1. Field of the Invention
The present invention relates to a semiconductor floorplan layout, and more particularly, to a semiconductor floorplan layout which integrates sections of a register renaming circuit of a superscalar RISC chip.
2. Related Art
Given instructions with two input operands and one output value, as holds for typical RISC (reduced instruction set computer) instructions, then there are five possible dependencies between any two instructions: two true dependencies, two anti-dependencies, and one output dependency. Furthermore, the number of dependencies between a group of instructions (such as a group of instructions in a window) varies with the square of the number of instructions in the group, because each instruction must be considered against every other instruction. Complexity is further multiplied by the number of instructions that the processor attempts to decode, issue, and complete in a single cycle, because these actions introduce dependencies, are controlled by dependencies, and remove dependencies from consideration.
True dependencies (sometimes called "flow dependencies" or "write-read" dependencies) are often grouped with anti-dependencies (also called "read-write" dependencies) and output dependencies (also called "write-write" dependencies) into a single group of instruction dependencies. The reason for this grouping is that each of these dependencies manifests itself through the use of registers or other storage locations. However, it is important to distinguish true dependencies from the other two. True dependencies represent the flow of data and information through a program. Anti- and output dependencies arise because, at different points in time, registers or other storage locations hold different values for different computations.
When instructions are issued in order and complete in order, there is a one-to-one correspondence between registers and values. At any given point in execution, a register identifier precisely identifies the value contained in the corresponding register. When instructions are issued out of order and complete out of order, the correspondence between registers and values breaks down, and values conflict for registers. This problem is severe when the goal of register allocation is to keep as many values in as few registers as possible. Keeping a large number of values in a small number of registers creates a large number of conflicts when the execution order is changed from the order assumed by the register allocator.
Anti- and output dependencies are more properly called "storage conflicts" because the reuse of storage locations (including registers) causes instructions to interfere with one another even though the conflicting instructions are otherwise independent. Storage conflicts constrain instruction issue and reduce performance.
In view of the above discussion it becomes clear that implementing data dependency circuits, and register renaming circuits in general, is complex and requires a great deal of semiconductor area. Superscalar RISC processors in particular, strive to simultaneously execute multiple instructions. As this technology develops, chip developers attempt to simultaneously execute more and more instructions. Thus, the required amount of dependency checking increases at an exponential rate. What is needed is a layout technique (also called a floorplan) that can integrate sections of the register renaming circuit (RRC) to conserve semiconductor area.
A more detailed description of some of the basic concepts discussed in this application is found in a number of references, including Mike Johnson, Superscalar Microprocessor Design (Prentice-Hall, Inc., Englewood Cliffs, N.J., 1991); John L. Hennessy etal., Computer Architecture-A Quantitative Approach (Morgan Kaufmann Publishers, Inc., San Mateo, Calif., 1990). Johnson's text, particularly Chapters 2, 6 and 7 provide an excellent discussion of the register renaming and data dependency issues addressed by the present invention.